1. Field of the Invention
The present invention relates to a cascade scanning optical system having a pair of laser scanning optical systems which are arranged along the main scanning direction and controlled to operate in synchronization with each other so as to realize a wide scanning line. More specifically the present invention relates to an apparatus of such a cascade scanning optical system, having a pair of laser scanning optical systems, for synchronizing the rotation of a polygon mirror of one laser scanning optical system with the rotation of a polygon mirror of the other laser scanning optical system, to prevent a pair of scanning lines that are to be aligned, respectively generated by the pair of laser scanning optical systems, from being deviated from each other in the sub-scanning direction.
2. Description of the Related Art
A cascade scanning optical system having a plurality of laser scanning optical systems arranged along the main scanning direction to realize a wide scanning line is known. Such a type of scanning optical system is disclosed in Japanese Laid-Open Patent Publication No. 61-11720, published on Jan. 20, 1986. This publication discloses a cascade scanning optical system having a pair of laser scanning optical systems each having a laser beam emitter, a polygon mirror serving as a deflecting device, an f.theta. lens, etc. The pair of laser scanning optical systems are synchronously driven to emit respective scanning laser beams to a photoconductive surface (scanning surface) of a photoconductive drum on a common line thereon extending in parallel to the axial direction of the photoconductive drum. The pair of scanning laser beams respectively scan two adjacent ranges of the common line on the photoconductive surface so as to scan the photoconductive surface of the photoconductive drum in the main scanning direction in a wide range.
There is a fundamental problem to be overcome in such a type of cascade scanning optical system. Namely, how can a scanning line, made on the photoconductive drum by the scanning laser beam emitted from one laser scanning optical system of the cascade scanning optical system, be accurately aligned with another scanning line, made on the photoconductive drum by the scanning laser beam emitted from another laser scanning optical system of the cascade scanning optical system, so that the scanning lines are not apart from each other in either the main scanning direction or the sub-scanning direction, i.e., so as to form a straight and continuous scanning line through the combination of the separate scanning lines.
It is sometimes the case that each reflecting surface (scanning laser beam deflecting surface) of a polygon mirror used in the cascade scanning optical system slightly tilts from its original position. In the case where the angle of each reflecting surface of the polygon mirror of one laser scanning optical system is different from that of the other corresponding laser scanning optical system, the pair of scanning lines, which are respectively generated by the aforementioned corresponding reflecting surfaces forming a straight and continuous scanning line, will deviate from each other in the sub-scanning direction on the photoconductive drum. This results in a gap or deviation occurring between the two scanning lines in the sub-scanning direction, so that a straight and continuous scanning line will not be formed. A similar problem will arise in the case where one or both of the polygon mirrors rotate with a tremor or oscillation.